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Egnos Inmarsat Error Correction

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Tonight I was on a hike and wanted to waymark a location. I was about to switch on the averaging to get a better fix, when I saw that the estimated accuracy (estimated error?) was 7.6 feet.


This was rather stunning as I'm more used to seeing an accuracy of 20 to 40 feet. I switched to the satellite page and found capital "D"s showing, indicating WAAS/EGNOS error corrections. "New" satellites 44 and 39 were shown.


There have been a few occasions when I have seen error correction for a few minutes at a time, but this stayed consistent for at least an hour. I assume that one of these satellites 44 and 39 is the geo-stationary east atlantic InMarSat. As far as I know, that is received in Scotland, Ireland and parts of Wales and Cornwall but the further south you go, the less coverage you get. The other InMarSat transmitting the EGNOS signal is over the Indian Ocean and should be completely out of my range. I live in north east England in Teesside.


So the questions are: Which areas of the UK regularly receive GPS error corrections, and where are satellites 44 and 39?

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Thanks, Phoenix. I did. I now know that the EGNOS signal I see very occasionally is satellite 33 (Atlantic Ocean Region -East). The ones I saw for at least an hour are satellite 39 (Indian Ocean Region - West) and 44 (Indian Ocean Region - East).


I also found out why my sister, who has an identical GPSr, couldn't get error correction when she was walking beside me. It takes about 5 minutes for the GPSr to grab the almanac. If it loses signal, it will take another 5 minutes etc. I always have WAAS enabled. She only enabled it when I suggested it.


What I still can't find out from previous posts is the answer to the first question. I get the feeling that in the UK, it is rare to receive error corrections. I could be wrong. Possibly the Scots and Irish use it all the time?

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What I still can't find out from previous posts is the answer to the first question.  I get the feeling that in the UK, it is rare to receive error corrections.  I could be wrong.  Possibly the Scots and Irish use it all the time?

As of this moment, there are several factors which determine whether EGNOS corrections can be seen, and applied, or not.


The make and model of receiver is important, The EGNOS signal includes a "do not use" indicator which does what its name implies. Some units respect this indicator, and won't apply the corrections - others (recent Magellan types, for example) ignore it, and attempt to use them.


The EGNOS system is in the final stages* of operational testing. This means that its signal mode may change, during the tests.


Local geography affects the ability of handheld units to receive the signal. All the SV's mentioned are fairly close to the horizon (from the British Isles), so reception may be marginal, or non-existent. Climbing a mountain helps. :unsure:


To answer your question: there is no evidence that differential corrections are more "available" in Scotland or Ireland - although both of them are good places to visit, for other reasons.



[* Source: ESA website: press release.]

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I get the feeling that in the UK, it is rare to receive error corrections. I could be wrong. Possibly the Scots and Irish use it all the time?

The AOR-E satellite is easily receivable from throughout the UK, unlike the IOR one.


In most of Scotland we cannot get a good signal from IOR because it is so close (or even below) the local horizon, but the elevation angles to AOR-E from Scotland and Ireland are comparable to England and quite satisfactory in most terrain.


Here are some of the elevation angles I calculated for the four places:

Edinburgh 25.4°

Glasgow 25.6°

Belfast 27.2°

Dublin 28.5°

Bristol 29.9°

London 29.4°


Any elevation angle above about 10-15° is quite useable, so long as you haven't got a local obstruction between you and the azimuth of the InMarSat bird.


Cheers, The Forester

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I have seen the magical D indicator once for approx. 1 minute on my Geko 201 in north County Durham one afternoon out of the 9 months I've had the receiver. I figured this was due to the fact that I was a. high up, b. on the right side of the hill! This includes usage in the Lake District, Scottish mountains and the Canary isles. Of course, my GPSr may be showing reception most of the time, it's just that I'm not looking at it!

All in all, it probably won't make that much difference in my ability to find caches!

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I have seen the magical D indicator once for approx. 1 minute on my Geko 201 in north County Durham one afternoon out of the 9 months I've had the receiver.

Garmins take into account a "use" / "do not use" flag which is transmitted by the system. Magellans do not.


For most of the of the EGNOS testbed programme the flag has been set to "do not use". For short periods the new Artemis satellite was transmitting corrections in "full" mode and then Garmin users were able to use the WAAS corrections.


The reason(s) why the two manufacturers have differing policies on the flag is a matter of speculation, but perhaps one reason might be that Garmin produces aviation versions of GPSr whereas Magellan have discontinued their General Aviation units. The Garmin GA units have an output signal which can be connected to some aircraft autopilot inputs which mimic the standard signals from Instrument Landing Systems (ILS). Thus it is quite possible to fly an automated ILS approach with a retail level GPSr, so it is understandable that Garmin is cagey about allowing users to ignore the do not use flag in view of how litigious relatives of deceased aviators can become.


One of the reasons why the testing programme has been so rigorous and so lengthily delayed has been the need to prove beyond reasonable doubt that the EGNOS system is fit for safety critical missions such as automated ILS-like landings of aircraft and dynamic positioning systems of ships engaged in delicate manouvres at close quarters.


For non-critical tasks such as hunting tupperware, the EGNOS system is already useable for us in the UK, even at Northern latitudes, whenever they are transmitting correction signals from the InMarSat III-F2 (AOR-E) satellite.

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No I get that too and no little D'a as yet.


I will miss these threads once this thing is up and running properly. You know..


Newbe says should I use WAAS.

Half replies say yes its wonderful I now get 3" accuracy and am always standing on top of the cache.

Other half say I've never got it to work.

Then someone says why don't I get it when I'm standing next to some one who has got it working.


And still we all manage to find caches!!! :)

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Is it just me that sees sat 33 change to 37 and back again on a regular basis?

The #33 satellite is the Inmarsat Atlantic Ocean Region East one. From St Neots it is an elevation angle of 28.7° on azimuth bearing of 199.1°


The #37 one is Artemis. When launched, the upper stage booster rocket failed and so the satellite was stuck in a completely useless orbit. By ingenious and patient use of a couple of thrusters and the reaction wheels in a mode for which they were never designed, the mission controllers have saved the craft and have managed to coax it up to the right altitude and have placed it in a geosynchronous but not geostationary orbit. It is now flying at a Longitude of approximately 21° 25'E, but its Latitude varies up to 3° 38.65' either side of the Equator. That puts her at an azimuth of about 154° from St Neots and an elevation angle which varies between about 23° and about 31°.


During the EGNOS trials they have been switching back and forth among these two satellites and the Indian Ocean Region one. IOR (#39 on a Garmin) is the satellite through which most of the EGNOS testing has been done, but it is not easily visible from your location, being a mere 6° or so above a level horizon and thus suffers a lot from terrain and other obstruction obscurement and has a raypath which punches almost sideways through the atmosphere and therefore loses most of it signal strength. For us in Southern/Central Scotland the elevation angle of IOR is a mere 3° which renders it quite useless for us in most geocaching environments.


Cheers, The Forester

Edited by The Forester
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Now thats what I call an explanation, cheers Forester, at least I know it's not the GPSr playing up.

I get the signals fairly regularly at low strength but the bars never go dark so I guess thats just the 'do not use' flag coming into play as it's a Garmin so I normally leave waas disabled and use the channels for other sats.

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